1. Field of the Invention
This invention relates generally to an apparatus and method for sensing physical phenomena and particularly to an optical sensing system and its associated rotation sensor head for sensing rotation of one or more rotatable objects. Still more particularly, the invention relates to a high accuracy optical radar, fiber optic rotation sensing system and its associated rotation sensor head for use on an aircraft in order to measure rotations of various moving parts of the aircraft at high rates and with short lag times and simultaneously processing optical signals from position sensor heads.
2. Background of the Related Art
Traditionally, electrical sensors are used to measure the rates of rotation of rods connected to rotors and position of various actuators in an aircraft. Results of these measurements are then fed back to a system flight controller which processes this information and outputs appropriate commands to control the rotors and actuators.
Rotor rotation rates can vary from a few hundred revolutions per minute (rpm) to a few tens of thousands of rpm depending on the rotor. A sensor head associated with a rod connected to the rotor sends a rotation signal representing the rate of rotation of the rod to a processor that calculates a rotation rate. Rotation measurements of the rod must be fed to the flight controllers at rates up to several hundred Hz, with a lag time less than a few milliseconds, and with accuracies of 0.1 to 1%. Here, lag time is defined to be the time between completion of raw data collection from a sensor and transmission of a position measurement to the flight controller.
While this is taking place, position measurements are also being made. A typical position actuator has a rod secured within an outer casing. Depending on the actuator, the rod can move back and forth a maximum distance of a few millimeters to over 50 cm. This maximum distance is often referred to as a stroke. A sensor head associated with the actuator sends a position signal representing the position of the actuator rod to the processor which in turn calculates a position measurement. Position measurements of the rod must be fed to the flight controller at rates up to several hundred Hz, with a lag time less than a few milliseconds to 0.5 ms, and accuracies of a fraction of a percent.
Fiber optic sensing systems offer numerous advantages over conventional electrical sensing systems. First, they are small and lightweight. In addition, they can be made immune from electromagnetic interference (EMI) which can occur near power lines, and electromagnetic pulses (EMP) which can occur in the event of a nuclear explosion. EMI/EMP immunity is an especially important advantage for new generation aircraft which have skins made largely of composite (non-metallic, non-shielding) material. Without heavy, bulky and expensive shielding of conventional electrical sensors and control lines, these next generation aircraft cannot be safely flown in areas of severe EMI/EMP. Therefore, "fly-by-light" systems or fiber optic sensing systems have the potential to replace "fly-by-wire" systems in future aircraft to measure rotation rates of various rotation rods in the aircraft.
Chopper wheel systems are one approach to measuring rotation rates of rods in an aircraft. Chopper wheel systems involve shining light through a chopper wheel and detecting the transmitted light at a photodetector. As the chopper wheel rotates, it chops the light before it reaches the photodetector causing a square wave signal to be output by the photodetector. This square wave signal output by the photodetector is then input to an amplifier and then applied to a zero crossing detector which counts the number of chops in a given period of time. Such a chopper wheel system, however, does not allow multiplexing of multiple rotation heads, because a separate photodetector is required for each sensor. Moreover, such a system does not support position sensor heads.
On the other hand, fiber optic (position) sensor systems do not have the capability to receive and process optical signals from both rotation sensor heads and position sensor heads. It is desirable that a single sensor system be used to measure rotation as well as position of the various actuators on an aircraft.
Some fiber optic position sensing systems use digital or optical encoding techniques in order to vary the amplitude of an incident optical signal as a surface is moved. However, sensor heads for these types of sensor systems cannot be easily multiplexed with each other and consequently cost, complexity, weight and volume of the system are increased. Furthermore such systems cannot simultaneously operate both rotation sensor heads and position sensor heads.
Another type of fiber optic position sensor system sometimes called an optical time domain reflectometer (OTDR) uses a pulsed optical source. In particular, OTDRs measure distances to in-line fiber reflectors by estimating the round trip transit time of a light pulse from the pulsed optical source to the in-line fiber reflector and back to a detector. Both the measurement accuracy and estimation times are fundamentally limited by the amplitude and width of the light pulse. Also, such a sensor system cannot be modified to measure rotation rates. In fact, it is even difficult to multiplex multiple position sensor heads in OTDR systems.
Another type of fiber optic position sensor system is a coherent optical frequency domain reflectometer (COFDR). COFDRs use coherent frequency modulated (FM) optical radiation. However, the rotation rates of rotating objects on aircraft are much smaller than the line width of optical sources used in the COFDR and consequently cannot be easily measured, and optical sources with narrow line widths tend to have low output power and low reliability. Also, Doppler shifts due to linear motion of associated actuators cause large shifts in intermediate frequency estimations which translates into large errors in position measurements. Moreover, all fibers used in COFDRs must be single mode polarization preserving fibers in order to coherently optically mix returned FM optical signals with an optical local oscillator signal and consequently are difficult to install and maintain.